About Cirsium undulatum (Nutt.) Spreng.
Cirsium undulatum (Nutt.) Spreng. is a species of thistle with the common names wavyleaf thistle and gray thistle. It is native to most of central and western North America, ranging from British Columbia east to Manitoba, and south to Durango, Mexico. It has also been recorded as an introduced species outside its native range. This thistle is widespread, and grows across a wide variety of habitat types. It is a perennial herb that grows over 200 cm (79 in) tall. If it branches at all, the stem branches only a few times near the top of the plant. Its leaves have very wavy edges, and are typically cut into shallow toothed lobes. The longest leaves, which are located near the base of the plant, can reach up to 30 centimetres (12 inches) in length. The inflorescence holds one or more flower heads, each up to 5 centimetres (2.0 inches) long and wide. Each flower head is lined with spiny phyllaries of varying shapes. The individual flowers within a head range in color from white to lavender to pink, and are up to 5 centimetres (2.0 inches) long. The fruit is an achene a few millimeters long, attached to a pappus that can reach up to 4 centimeters in length. The seed production of Cirsium undulatum is known to be reduced by the exotic weevil Larinus planus, which was originally released to control Canada thistle.
